1. Field of the Invention
The present invention concerns electronic switches having an input terminal and an output terminal by which they can be inserted into the power circuit of a load to be controlled and an electronic switch unit, for example a triac, between said input terminal and said output terminal, controlled by a control circuit under the control of an actuator that the user can operate on directly or indirectly.
2. Description of the Prior Art
At present the control circuit usually benefits from a particular power supply which, at the cost of appropriate additional wiring, is taken directly between live and neutral of the corresponding power circuit. In practice this is the case in most practical implementations commonly used.
Consideration has nevertheless been given to making the control circuit from the anode to the trigger of the electronic switch unit, which avoids all additional wiring and which has the further advantage of automatic triggering of the electronic switch unit when the current passes through zero.
This control circuit then inevitably includes a series current limiter resistor the value of which must be appropriate to procuring the required triggering and which for this reason is frequently called the trigger resistor.
At present received wisdom is that the value of the current limiter resistor must be relatively high, in practice in the order of 100 ohms.
In conjunction with the inevitable internal capacitance between the trigger and the anode of the electronic switch device the current limiter resistor constitutes a filter.
Just as inevitable is a resulting phase-shift between the current and the voltage.
If the value of the current limiter resistor is high, the angle of this phase-shift, commonly known as the conduction angle, is sufficiently high for the corresponding voltage to be high when the current passes through zero and thus on triggering.
This leads to the unwanted generation of spurious harmonics in the power circuit that can be a problem on the corresponding mains supply.
To avoid this problem, which is necessary to comply with standards in this area, it is currently standard practice to insert a smoothing coil in the power circuit.
However, apart from its non-negligible cost and the implied assembly costs, the smoothing coil has the major disadvantage of causing high heat dissipation.
When used in a confined space, for example inside a pattress, this heat dissipation leads in practice to a significant reduction in the rupture capacity of the electronic switch unit employed in order to comply with the limit temperature that is not to be exceeded.
The rupture capacity is usually halved.
Another solution known in itself is to replace the current limiter resistor with a capacitor.
This has the drawback of introducing a phase-shift of 90.degree. between the current and the voltage.
A general object of the present invention is an arrangement that avoids these drawbacks.
It is based on the previously unformulated observation that it is possible, without compromising operation, to choose the value of the current limiter resistor so that the conduction angle produces only acceptable spurious harmonics.